1. Field of the Invention
The present invention relates to a coaxial 2-of-n -relay transfer switch having reed contacts arranged in electrically conductive, non magnetic tubes, which contacts are arranged in two groups to each n reed contacts in a star shape in such a way that, per group, the one contact spring terminals meet in a branching point and, in a metallic head encompassing the branching point, are connected in common to a coaxial input line, and that the, in each case, other contact spring terminals of mutually corresponding reed contacts of both groups meet in pairs in second branching points and are there connected to output lines, and in which the metallic heads and the metallic tubes encompassing the reed contacts are so designed in their interior that, seen electrically, additional inductances are inserted on the one hand between the end of the inner conductor of the coaxial input line and the first branching point of the reed contacts and, on the other hand, between the first branching point and the contact points of the reed contacts.
2. Description of the Prior Art
In test devices for rapid digital switching circuits, it is often the task to connect, in a program-controlled fashion, one of n input terminals of a test object to the output of a generator and to the one input of an oscillograph via coaxial lines and transfer switches. The connection therein is to be done in such a fashion that down to signal rise times of 0.5 ns, the network formed in each case represents, to the greatest degree possible, a reflection-free, wide band 50 ohm coaxial connection of a specific electrical length. At the same time, a selected other one of the n input terminals of the test object must be able to be connected to the output of a second generator. The technical solution for this problem is usually to be found in relay matrices having two inputs and n outputs (for example, where n equals 64) which are composed of individual 2-of-n relay transfer switches connected in cascade (for example, three transfer switches in cascade with a branching number of four per stage).
FIG. 1 shows the principle wiring diagram of such a 2-of-4 relay transfer switch. Two coaxial input lines E1 and E2 can be connected via, in each case, one of the four relay-controlled reed contacts r1. . .r4 or, respectively, of the four relay-controlled reed contacts k1. . .k4, to one of four coaxial output lines in such a way that in both transmission directions it is possible to have pulse transmission which is as reflection-free as possible, with low distortion and low attenuation. For specifically this type of transfer switch, no practical solutions have as yet become known which lead to the desired wide band character.
A 2-of-4 transfer switch has become known in which, on the top side of a multi-layer printed circuit board, four reed contacts are arranged in a star shape and the branching point of the reed contacts on the top of the multi-layer printed circuit board is surrounded by a metallic head into which, from the underside of the multi-layer printed circuit board, a coaxial plug connection is inserted. On the same printed circuit board a second metallic head with four further reed contacts is arranged on the bottom side. The feed into this second head is accomplished from the top via a second coaxial plug connection which leads to the branching point of the second four reed contacts.
The two-times-four reed contacts are connected to coaxial output plug connections in that the connecting legs of corresponding reed contacts are connected to soldering eyes, and from the soldering eyes, 50 ohm strip lines lead to the coaxial output plug connections. In order to transfer the quasi-coaxial line, which the inside conductor forms together with the copper tube encompassing the reed contacts, as shock free as possible into the 50 ohm strip line in the region of the soldering eyes, the tube ends are connected via wires and other soldering eyes to the ground plane of the printed circuit board.
For the following reasons, arrangements of this type cannot be produced sufficiently low in reflection and wide band enough:
1. The necessary lateral displacement of the two heads, which is necessary to that the feed-in lines can be connected to the coaxial input plug connections, causes the connections of the reed contacts to the soldering eyes to be unequally formed and, therefore, only to a limited degree electrically equally long. PA1 2. The junctions from the reed contacts onto the 50 ohm strip lines with, in each case, a contact connecting wire and a zero-volt wire are shock-afflicted. PA1 3. The soldering eyes disturb the lines with a capacity of about 2 pF against zero volts. PA1 4. The junctions from the 50 ohm strip lines to the output sockets are shock-afflicted.
For this reason, fundamental reflections &gt;20% (at 150 ps generator rise time), a bandwidth of &lt;1 GHz and residual time constants &lt;500 ps are to be expected; and the transit time differences of the individual paths lie above 20 ps.